The present invention relates to a semiconductor light emitting device including a layer of gallium nitride based compound semiconductor for emitting the light in the bluish color (ultraviolet to yellow) and a method of manufacturing thereof, or more in particular to a semiconductor light emitting device and a method of manufacturing thereof with an improved bonding strength of the wire bonding and the ohmic contact of the electrode provided on an n-type layer.
The conventional semiconductor light emitting device for emitting the light in bluish color has a structure shown in FIG. 3, for example. Specifically, on a sapphire substrate 21, a low-temperature buffer layer 22 composed of n-type GaN, an n-type layer (clad layer) 23 with GaN epitaxially grown at high temperature, an active layer (light emitting layer) 24 composed of a material such as InGaN based compound semiconductor (meaning that the compound crystal ratio between In and Ga is variable in many ways, which is true also in the description that follows) which is determined by the desired light emitting wavelength and in such a manner that the band gap energy is smaller than that of the clad layer, and a p-type layer (clad layer) 25 composed of p-type GaN. A p-side electrode 28 is formed on the surface of this semiconductor lamination, and an n-side electrode 29 is formed on the surface of the n-type layer 23 exposed by etching off a part of the semiconductor lamination. In order to improve the effect of confining the carriers, the n-type layer 23 and the p-type layer 25 may be formed of a compound semiconductor layer of AlGaN group (meaning that the ratio between Al and Ga is variable, as in the description that follows) on the active layer 23.
With this structure, the n-type electrode 29, as described in Japanese Patent Publication Unexamined No. HEI 7-45867 and Japanese Patent Publication Unexamined No. HEI 7-254733, is formed of a Tixe2x80x94Al alloy from the viewpoint of the ohmic contact with the n-type layer or a Tixe2x80x94Au alloy for preventing the bonding strength from being decreased in view of the fact that Al is easily oxidated, or as described in Japanese Patent Publication Unexamined No. HEI 8-274372 and as shown in FIG. 3, formed of an alloyed lamination of an Al layer, a Ti layer and an Au layer. Further, the p-side electrode 28, as described in Japanese Patent Publication Unexamined No. HEI 8-274372, for example, is formed of a transparent electrode layer (diffusion metal layer) with a lamination of a Ti layer and a Ni layer and an electrode constituting a bonding pad with a lamination of a Ni layer and an Au layer, which are alloyed simultaneously with the n-side electrode.
As described above, when emphasizing the ohmic contact with the n-type layer, the n-side electrode is formed of an Alxe2x80x94Ti alloy, and from the viewpoint of wire bonding, the surface thereof is formed of Au, and this laminated structure is alloyed by heat treatment. However, as described in Japanese Patent Publication Unexamined No. HEI 8-274372, for example, the laminatedly arranged metals are patterned at a time. Thus, Al and Ti formed at the lower part are exposed from the patterned sides. Further, in the case where the Au layer is subjected to heat treatment in contact with Al or Ti for alloying, Al or Ti is diffused in the Au layer and also deposited on the surface of the Au layer. When Al or Ti is exposed, on the other hand, they are liable to be easily oxidated and corroded by water. The problem, therefore, is that aluminum or the like alloyed and exposed by diffusion on the surface of Au or the sides of the electrode are corroded by the water produced from the mold resin covering the surrounding and the reliability is reduced. Further, the deposition and oxidation of Al which may occur on the surface of the Au layer reduces the adhesive power of the wire bonding and deteriorates the reliability of the wire bonding while at the same time reducing the yield. In this way, the conventional semiconductor light emitting device using a gallium nitride based compound semiconductor is liable to pose the problem of the wire bonding and the ohmic characteristic of the n-side electrode.
The present invention has been developed to solve this problem, and an object of the invention is to provide a semiconductor light emitting device of a gallium nitride based compound semiconductor having an electrode structure superior in both ohmic contact characteristic and wire bonding characteristic.
Another object of the invention is to provide a method of manufacturing a semiconductor light emitting device by which the electrode structure described above can be formed without increasing the manufacturing steps.
Still another object of the invention is to provide a method of manufacturing a semiconductor light emitting device in which the reliability is not reduced by the diffusion of another metal such as Al into the metal of a bonding electrode.
According to the present invention, there is provided a semiconductor light emitting device comprising a substrate, a semiconductor lamination forming a light emitting region including an n-type layer and a p-type layer of gallium nitride based compound semiconductor, and an n-side electrode and a p-side electrode electrically connected to the n-type layer and the p-type layer, respectively, wherein the n-side electrode is formed of an ohmic contact electrode and a bonding electrode, and the bonding electrode covers the surface and the sides of the ohmic contact electrode.
The gallium nitride based compound semiconductor is a semiconductor composed of a compound of the III-group element Ga and the V-group element N, the III-group element Ga partially or wholly replaced by another III-group element such as Al or In, and/or a compound of the V-group element N partially replaced with another V-group element such as P or As.
With this structure, the ohmic contact electrode is completely covered by the bonding electrode, and therefore Al, etc. is not exposed. Even in the case where the surrounding of the electrode is covered with a molding resin, therefore, the electrode is not corroded by the moisture of the resin or the like, and a sufficient reliability is maintained.
At least the outer surface of the bonding electrode is preferably formed of an Au layer from the viewpoint of the corrosion resistance and the bonding characteristic.
The bonding electrode and the p-side electrode, which are formed of the same material, can be efficiently formed at the same time. For example, these component parts are formed of a laminated structure of Ti and Au.
In the case where the contact electrode is made of a Tixe2x80x94Al alloy, a superior ohmic contact is obtained with the n-type layer.
Specifically, the substrate is formed of sapphire, the portion of the semiconductor lamination forming a light emitting region is made of an InGaN based compound semiconductor sandwitched between the n-type and p-type layers of a gallium nitride based compound semiconductor thereby to form a bluish corol light emitting semiconductor device.
More specifically, the n-type layer is provided nearer to the substrate, and the n-side electrode is formed on the surface of the n-type layer exposed by etching off part of the semiconductor lamination. A diffusion metal layer (transparent electrode) is formed on the surface of the semiconductor lamination. The p-side electrode is formed on the diffusion metal layer. Both the diffusion metal layer and the ohmic contact electrode of the n-side electrode are formed of an alloy layer.
A method of manufacturing a semiconductor light emitting device according to the present invention is characterized in that a semiconductor layer forming a light emitting region including an n-type layer and a p-type layer of gallium nitride based compound semiconductor on a substrate is grown thereby to form a semiconductor lamination. A p-side electrode and an n-side electrode are formed and electrically connected to the p-type layer and the n-type layer, respectively. In the process, an ohmic contact electrode is formed on the surface of the n-type layer, and then a bonding electrode is formed in such a manner as to cover the ohmic contact electrode, while at the same time forming the p-side electrode with the same material and by the same process as the bonding electrode.
By forming the bonding electrode and the p-side electrode of the same material at the same time in this way, the number of manufacturing steps is not increased even in the case where the n-side electrode is divided into the contact portion and bonding protion.
Specifically, an n-type layer, an active layer and a p-type layer are sequentially formed on a substrate thereby to form a semiconductor lamination. By removing a part of the semiconductor lamination, the n-type layer is exposed, and the n-side electrode is formed on the surface of the n-type layer thus exposed.
A metal layer is formed on the semiconductor lamination and alloyed thereby to form a diffusion metal layer. The p-side electrode is formed on the surface of the diffusion metal layer. The contact electrode of the n-side electrode is formed by depositing and alloying metal layers on the n-layer. The process of alloying the diffusion metal layer and the process of alloying the contact electrode are desirably carried out at the same time to minimize the number of steps.
More specifically, a Ti layer and an Al layer for forming the contact electrode are deposited, and a Ni layer and an Au layer for forming the diffusion metal layer are deposited. The Tixe2x80x94Al layers and Nixe2x80x94Au layers are heat-treated at the same time for alloying them, respectively. After that, metal films for the bonding electrode and the p-side electrode can be formed at the same time. By doing so, the heat treatment is not required after forming the bonding electrode, and the bonding electrode is not diffused with Al or the like. As a result, the surface of the bonding electrode can be kept clean, and the reliability of wire bonding is not adversely affected or no corrosion by penetration of moisture occurs.